Thrombin is an important serine protease in hemostasis and thrombosis. One of the key actions of thrombin is receptor activation. A functional human thrombin receptor (TR), cloned by Coughlin in 1991 (T.-K. Vu, Cell 1991, 64, 1057), was found to be a member of the G-protein coupled receptor (GPCR) superfamily. The receptor activation putatively occurs by N-terminal recognition and proteolytic cleavage at the Arg-41/Ser-42 peptide bond to reveal a truncated N-terminus. This new receptor sequence, which has an SFLLRN (Ser-Phe-Leu-Leu-Arg-Asn SEQ. ID. No. 1) N-terminus acting as a tethered ligand to recognize a site on the receptor, can trigger activation and signal transduction leading to platelet aggregation. Peptide analogues based on this hexapeptide have also shown good agonist activity leading to platelet aggregation. Since 1991, two other protease-activated receptors with extensive homology to the thrombin receptor, “PAR-2” and “PAR-3,” were cloned, and found to be activated by similar N-terminal hexapeptide sequences. Hence, agonists/antagonists of the thrombin receptor, such as those included in this invention, may be useful in activating/antagonizing these protease-activated receptors as well.
Activation of the thrombin receptor by agonist compounds of this invention may mimic thrombins role in tissue repair. Thrombin can initiate effects related to wound healing, such as: increasing vascular permeability to allow entry of cells and fluid into injured tissue (A. B. Malik, Semin. Thromb. Haemostasis 1986, 184); increasing the synthesis of PDGF by endothelial cells (J. M. Harlan, J. Cell Biol. 1986, 103, 1129); and increasing the adhesion of platelets, monocytes, and neutrophils to endothelial cells (M. P. Bevilacqua, Science 1989, 243, 1160). Tissue repair in rats following surgical incision is accelerated by the use of thrombin (D. H. Carney, J. Clin. Invest. 1992, 89, 1469). Thus, agonists at the thrombin receptor may be useful as wound healing agents or in tissue repair.
The peptide-based antagonists of the thrombin receptor in this present invention may show efficacy against myocardial infarction, stroke, restenosis, angina, atherosclerosis, and ischemic attacks by virtue of their ability to prevent platelet aggregation. The thrombin receptor has also been identified on other cell types: endothelial, fibroblast, osteosarcoma, smooth muscle, and neuronal/glia. Thrombin activation of endothelial cells upregulates P-selectin to induce polymorphonuclear leukocyte adhesion—an inflammatory response of the vessel wall (Y. Sugama, J. Cell Biol. 1992, 119, 935). In fibroblasts, thrombin receptor activation induces proliferation and transmission of mitogenic signals (D. T. Hung, J. Cell Biol. 1992, 116, 827). Thrombin has been implicated in osteoblast proliferation through its activation of osteoblast cells (D. N. Tatakis, Biochem. Biophys. Res. Commun. 1991, 174,181). Thrombin has been implicated in the regulation and retraction of neurons (K. Jalink, J. Cell. Biol. 1992, 118, 411). Therefore, in this context, the antagonist compounds of this invention may also be useful against inflammation, restenosis, cancer, osteoporosis, and neurodegenerative disorders.